1. Field of the Invention
The invention pertains to the field of semiconductor devices and to the field of data communication systems and data communication networks.
2. Description of Related Art
A steadily increasing flow of information requires improvements to existing technology of data transport and development of new devices and systems. Transporting signals at 10 Gb/s over a single-mode optical fiber has become a technology of the past. Transporting 40 Gb/s over a single-mode fiber for 100 km is an advanced technology that is becoming readily available. At 40 Gb/s, half-a-million simultaneous telephone conversations can be transmitted. Transporting above 40 Gb/s is the next challenge.
Advances in laser and optoelectronic device technology have made it possible to transmit more than one wavelength in the same fiber. This practice is known as wavelength division multiplexing (WDM). Adding wavelengths in the same fiber effectively increases the bandwidth capacity of a fiber and thus negates the immediate need to install additional fibers or increase the data bit rate to extremely high levels. In the full low-loss wavelength range of a single mode fiber (1.2–1.6 μm), some 1000 wavelength channels separated by 50 GHz may be used. At 40 Gb/s per wavelength, a total aggregate bandwidth of 40 Tb/s per fiber may be achieved (S. V. Kartalopoulos; “Introduction to DWDM Technology. Data in a Rainbow”, Wiley Interscience, New York (2000)).
Typically, WDM systems, or dense wavelength division multiplexing (DWDM) systems, used in long-haul and metropolitan area applications are based on expensive single lateral and longitudinal mode telecom transmitters. Wavelength tunable lasers offer a promising advantage. For DWDM applications, tunable lasers are advantageous because they provide laser switching between different channels, thus reducing the number of expensive devices and simplifying DWDM protocols (S. V. Kartalopoulos; “Introduction to DWDM Technology. Data in a Rainbow”, Wiley-Interscience, New York (2000)). Tunable lasers operating at or near 1.55 μm are currently used and in the future the whole 1.2–1.7 μm range is likely to be covered to provide sufficient bandwidth.
Traditional wavelength-tunable lasers are very expensive. They also require precise wavelength stabilization, which is usually achieved by using sophisticated temperature control and a feedback detection system to provide wavelength locking for each device. Using tunable laser arrays, and, in particular, arrays of vertical-cavity surface-emitting lasers (VCSELs), may reduce the cost of DWDM systems, as the production cost for a single laser channel in the array is much smaller than the cost of an individual laser. The costs of packaging, optical alignment, focusing, temperature and stabilization, which compose the major production costs of the device, do not scale with the number of the devices in the array contributing only once. Thus, tremendous cost reduction is possible. However, DWDM applications require devices with different and well-defined wavelengths, which is not normally possible for conventional single-chip VCSELs. For the full low-loss range of a single mode fiber (1.2–1.6 μm), some 1000 wavelength channels separated by 50 GHz may be used. At 40 Gb/s per single wavelength channel, a total aggregate bandwidth of 40 Tb/s per fiber may be achieved. For example, DWDM standard (ITU-T Recommendation G.692) defines 43 wavelength channels from 1530 to 1565 nm, with a spacing of 100 GHz, each channel carrying an OC-192 signal at 10 Gb/s (S. V. Kartalopoulos; “Introduction to DWDM Technology. Data in a Rainbow”, Wiley-Interscience, New York (2000)). Thus, a typical spacing of 0.8 nm between channels is required for 100 GHz.
Currently, wavelength-adjustable intelligent WDM and DWDM systems do not exist. The standard DWDM approach requires a precisely fixed wavelength. The only possibility to use wavelength-tunable lasers is to reduce inventory of fixed-wavelength lasers. The system itself always remained the same: many different fixed-wavelength light beams from different light sources are coupled to a single fiber (multiplexing) and separated at the exit of the fiber into different channels (demultiplexing), each channel operating with a separate photodetector. All the presently existing wavelength multiplexing and demultiplexing approaches are based on a precisely fixed wavelength of each DWDM channel. This makes DWDM systems very expensive.
Currently existing wavelength-tunable lasers may be edge-emitting lasers or VCSELs. Edge-emitting devices are conventionally fabricated as distributed-feedback lasers to ensure single longitudinal mode operation. Wavelength tuning by tuning the refractive index can be applied to these devices. This tuning can be achieved, for example, by a heat sink temperature change. Modulation of the refractive index may be caused by an electron-hole plasma effect due to the changing concentration of nonequilibrium carriers in the specially introduced distributed feedback (DFB) section. A DFB mechanism can be provided, for example, by etching a grating on the surface of the epiwafer, followed by subsequent overgrowth. For the same period of the grating, a change in the refractive index causes wavelength shift of the DFB modes. Usually different sections of the same in-plane waveguide structure are used in a real device.
Another way to achieve wavelength tunability in both edge-emitting and surface emitting lasers is to use external mirrors or diffraction gratings. Here, the tuning is realized by mechanical tuning of the effective cavity length of the device, or by angle adjustment of the diffraction grating mirror affecting the wavelength of peak reflectivity. In VCSELs, tuning of the cavity length may be realized by using different micro-electromechanical systems.
A disadvantage of both types of conventional tunable lasers is a long tuning time. In one case, the rate is limited by the time for tuning the heat-sink temperature, or the electron-hole plasma concentration. In the other case, the slow rate is related to the mechanical nature of the external mirror adjustment mechanism used. Frequency modulation signal transmission systems are generally not possible using these approaches.
Mechanically tunable lasers also suffer from various detuning mechanisms caused by material aging, humidity, and dirt absorption at gratings or suspended tunable mirrors. Vibrations can cause errors. Techniques to maintain wavelength stability (wavelength locking) are necessary for each of the separate devices, even in the case where laser arrays are used. If a wavelength-locking mechanism is applied to each of the devices in the array, it is more difficult to create cost-efficient systems.
There is a need in the art for improved wavelength tunable lasers and photodetectors and their application to novel wavelength division multiplexing systems.